An instrument which detects, measures, and indicates ionizing radiation such as alpha particles. It incorporates a Geiger tube (1) to detect and quantify the radiation, and its indications may be visual and/or auditory. Also called Geiger-Müller counter, Geiger-Müller tube (2), Geiger tube (2), Geiger counter tube (2), or Geiger-Müller counter tube (2).